Antihelium: the Heaviest Antimatter Particle Ever Detected

A particle accelerator experiment at the Brookhaven National Laboratory on Long Island, N.Y., has found antihelium-4, the heaviest antimatter that physicists have ever observed. The discovery could be one of the keys to figuring out why our universe in dominated by matter rather than antimatter—a scientific quest that continues this Friday with a new instrument that will launch on the space shuttle.

Scientists at Brookhaven National Laboratory on Long Island, N.Y., have discovered the heaviest antimatter ever observed. It's antihelium-4, a conglomeration of two antiprotons and two antineutrons. This antihelium is the antimatter partner to what physicists call the alpha particle (identical to a standard helium nucleus), and its discovery comes on the 100th anniversary of the discovery of the alpha particle itself, says Aihong Tang, lead author on the paper published in Nature.

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Scientists shot two streams of gold ions at each other at nearly the speed of light. The gold ion particle beams exploded into half a trillion pieces on contact. It took almost one billion particle smash-ups with Brookhaven's Relativistic Heavy Ion Collider—each collision happening in just ten-trillionths of a trillionth of a second—for the scientists to observe just 18 antihelium-4 particles. That may not seem like a lot, but 18 can be plenty in the world of high-energy physics. "Finding 18 of them means that it's way beyond the possibility of chance," Tang says, meaning they are sure that the signal is coming from the annihilation of antihelium-4 and not some other particle-collision event.

The discovery broke Brookhaven's own record for heaviest antimatter ever created, which was antihelium-3, consisting of two antiprotons and one antineutron. The new finding was especially remarkable because the heavier antimatter is, the less likely it will be created. High-energy particle collisions knock loose all the quarks and anti-quarks that make up matter and antimatter, respectively. But creating antihelium-4 requires 12 antiquarks—three for each antineutron and three for each antiproton—to combine in just the right way to produce a stable nucleus. Thanks to its more complex makeup (and the fact that even stable forms of antimatter will collide with matter and disappear), antihelium-4 is vanishingly rare and hard to find.

Antihelium isn't just fascinating and fleeting, though—it might also help scientists figure out some details about the dawn of our universe. The Relativistic Heavy Ion Collider where physicists found this new antihelium-4 is part of an experiment called STAR, an international collaboration to re-create conditions of the universe moments after the Big Bang. At that time, the thinking goes, the primordial soup was equal parts matter and antimatter, which annihilate each other on contact. But antimatter quickly gave way to matter, which arranged itself into the atoms and molecules that make up most of our universe today. Why there is more stuff than anti-stuff in the universe is one of the great lingering, vexing questions in physics.

Still, antimatter is around: Scientists have seen antielectrons (also called positrons) in thunderstorm clouds here on Earth, and antimatter is present wherever stars are being formed. This antimatter out in space, combined with the discovery of antihelium in the lab, could begin to provide some answers about antimatter. An instrument called the Alpha Magnetic Spectrometer (AMS), scheduled to be launched aboard the space shuttle Endeavor this Friday, is designed to detect distant cosmic sources of antimatter. The recent observation of antihelium-4 gives the AMS team a background rate on how often the stuff is formed. It's rare enough to be extremely unlikely to randomly happen on Earth, so if the instrument detects even a single antihelium-4 event, scientists are now more sure than ever that it would have to have traveled from the far reaches of the cosmos. "Now we know that if we see an anti-alpha from AMS, it won't be from Earth," Tang says.

If the space-based detector comes up with an antihelium nucleus, that will indicate the existence of an "antimatter area" somewhere in the universe, according to a lecture by Francesca Spada, a collaborator on the AMS project. That's what most physicists are hoping for, because the Standard Model, which is the rule book for how particles interact, predicts that there ought to be some antimatter out there at the edge of our matter-dominated universe. But if AMS sees no antimatter, then there could be a problem with the Standard Model. (Uh-oh.)

As for whether scientists will be able to discover heavier antimatter particles in the future, Tang says, don't hold your breath. Current detector technology is not sensitive enough to find anything heavier than the antihelium-4. The heavier antimatter particles are, the rarer they are, and the next stable antiparticle—antilithium-6, with three antiprotons and three antineutrons—is just too uncommon for current detectors to find. "Of course, we'll keep looking," Tang says.